Fuel cells provide electricity from chemical oxidation-reduction reactions and possess significant advantages over other forms of power generation in terms of cleanliness and efficiency. Typically, fuel cells employ hydrogen as the fuel and oxygen as the oxidizing agent. The power generation is proportional to the consumption rate of the reactants.
A significant disadvantage which inhibits the wider use of fuel cells is the lack of a widespread hydrogen infrastructure. Hydrogen has a relatively low volumetric energy density and is more difficult to store and transport than the hydrocarbon fuels currently used in most power generation systems. One way to overcome this difficulty is the use of reformers to convert the hydrocarbons to a hydrogen rich gas stream which can be used as a feed for fuel cells.
Hydrocarbon-based fuels, such as natural gas, LPG, gasoline, and diesel, require conversion processes to be used as fuel sources for most fuel cells. Current art uses multi-step processes combining an initial conversion process with several clean-up processes. The initial process is most often steam reforming (SR), autothermal reforming (ATR), catalytic partial oxidation (CPOX), or non-catalytic partial oxidation (POX). The cleanup processes are usually comprised of a combination of desulfurization, high temperature water-gas shift, low temperature water-gas shift, selective CO oxidation, or selective CO methanation. Alternative processes include hydrogen selective membrane reactors and filters.
A combustor, such as an anode tailgas oxidizer (ATO), is a crucial component for fuel processing systems. It combusts reformate, anode tailgas from fuel cells, or pressure swing adsorption unit off-gas to generate heat for reforming systems. All of these gases usually contain a certain amount of hydrogen. For example, reformate is largely a mix of hydrogen and carbon monoxide resulting as the product from the reforming of hydrocarbon feedstocks. Other constituents may include carbon dioxide, steam, nitrogen, and unconverted feedstock.
In addition to burning these gases, a combustor is also required to have the capability of burning fuels like natural gas or propane, especially during the initial start-up of the system.
A combustor could be a single catalytic type combustor. Although catalytic combustors have the advantages of relatively low combustion temperature and clean exhaust (less nitrogen oxides in it) compared to conventional flame type burners, the catalyst beds of catalytic combustors usually need to be preheated for start-up or fuels (e.g. natural gas) need to be preheated to a certain temperature before the combustor can be lit-off. As one option, an electric surface heater can be used to preheat the catalyst bed or natural gas fuel during start-up. In this manner, it usually takes at least 30 minutes to reach the light-off temperature for natural gas. As a result, quite a bit of electric energy (parasitic power) is consumed. Also, due to the fact that the preheating of fuels or combustion air was not incorporated in the design, a catalytic combustor has the difficulty of burning larger amounts of natural gas. Loss of flame frequently occurs due to the relatively slow flame speed of natural gas as compared to its higher superficial velocity at a larger flow rate.
Another problem associated with a common catalytic combustor is that the good mixing of reformate (specifically hydrogen) with air is required, and most of the time happens, outside the combustion zone. This mixing could cause potential safety problems due to the presence of formed hydrogen-air mixtures at their low flammable (or explosive) limit.
To overcome the aforementioned problems associated with a single catalytic combustor, a single flame burner could be used. Flame type burners typically use a spark ignitor to light-off fuels and do not require preheating of fuels (e.g. natural gas) for light-off. Also, unlike catalytic combustors, flame type burners do not require strong pre-mixing of fuels with the combustion air. Rather, fuels can light-off easily with appropriate stoichiometry at normal temperature. However, a flame type burner has to be ignited at a relatively fuel rich condition (i.e., lower oxygen/carbon ratio), thus its combustion temperature is usually higher unless a large amount of secondary air is introduced to dilute the flame. Due to the higher combustion temperature in a flame type burner, it is most likely to form nitrogen oxides in its exhaust in addition to carbon soot. Thus, a single flame burner is neither a long term viable solution nor an ideal solution in terms of the protection of environmental quality. The present invention provides a viable solution to the challenges associated with a catalytic combustor.